The Default From Email Address is the address for the FSRM server (which can be an ordinary mailbox built earlier). When the FSRM server wants to send an email of notification (perhaps because someone oversteps their quota or places an incorrect file type into a folder) it will use this email address. The server authenticates using its computer account and then submits the email to exchange. So for this to work you need to allow the FSRM to send the email on the mailbox behalf. You can do this by typing the following cmd in powershell on or connected to the Exchange server: